Corvette rotors on 69-70 Canadian full size Pontiacs.
A few years back, upon starting my chassis restoration, I had decided that my 69' Parisienne would need disc brakes in place of drums on the front. A little more performace would demand that braking be increased as well. Remebering my younger days, and some of the "near misses" I had in my 69' "drum" 2+2 as a result of brake fade, made this conversion a no brainer.It was what seemed pretty straightforward, and many had done it.Purchacing a 1970 Impala setup off ebay was my starting point. Unfortunately the rotors that came with it were too far gone to salvage. So I figured I'd just pop down to my local parts store and pick up a set... Wrong.Turns out the rotors were now obsolete! Perhaps due to the fact that this rotor design was used for two years, and, only used on the B body Chevrolets and Canadian Pontiacs 69 - 70. And standard only in 1970 on Impalas and Canadian built B Bodys. My parts guy attempted to go nation wide to find some, and thought he had hooked onto the last few new "integral hub" that were made available just a few years prior. Alas, they were gone, and any else had seemed to have dried up. Classic Industries actually was advertising them at the time, but they had none in stock, and nothing in the pipe. It's now said the stock discs are now available as of Jan 2008. One would, from time to time see a nos pair on ebay, and as expected, bidding would be frantic, with the price going as high as $500. Good used were very seldom seen, as people likely were hanging on to what they had. Rotors used in the salt belt were long gone. I could have bid crazy money, but I started probing around a little in the hope a alternate could be used.Some mention of a Vette rotor being used on the 69 -70 spindle was out on the web, but in little detail to date.Poking around, I soon discovered that 1965-82 (C3) Vette rotors had the same 5 x 4 3/4 bolt pattern as the big Chev and shared the same diameter (11.75") and thickness (1.25"). The offset looked obviously different, and the hat was taller, but then it dawned on me, what about mounting it OVER the Impala hub, instead of behind!My parts guy had a pair of rear rotors for a 73 Vette. I knew 65-82 were the same, but what about front to rear? Identical! Except for two access holes in the rears, to provide parking brake adjustment on the Corvette. One could use the rears on the front, no issues. So I took one home to try it out on a spindle I had in a mock up stand.Low and behold, It fit!
The original ebay Rotor and hub.
Stock vs Vette
Drum hub on spindle
Here's what I found.
First off, what I discovered right away, was that putting the vette rotor over the stock disc hub showed a slightly loose fit. The ID of the Vette rotor hat flange is slightly larger than the hub to wheel/rotor mating surface. Perhaps the Vette rotor on a Vette hub shows the same clearance anyway. The same loose fit is evedenced with the drum hub described below as well. But the hub flange did fit perfectly flat in the Vette rotor.
Some numbers;Vette rotor flange opening ID---- = 2.820".Impala hub OD mating suface = 2.780", a difference of .040.The Vette rotor hat face is .291" (9/32) thick.
Vette rotor fits over hub no clearance issues here.
My original thinking was that since the rotor was not centering on the hub, and bears all it's force strictly on the wheel studs, that more had to be done to bolster it. What I did was to open up those rotor "adjustment holes" to accept some cut down allen bolts, and drilled and tapped the hub to take the bolts. The rotor would now bear on the allen heads. My thinking now is that this was overkill, and people run these discs with no issues so far.
Allen bolt into opened up "access hole". Rear C3 rotor.
The second major thing I've discovered is that the drum brake hub will work on our spindle! It turns out it brings the inrease in track back in 1/8" compared to using a "Disc" hub. Putting the Vette rotor over the disc hub naturally increases the track by the thickness of the rotors flange.. to the tune of better than 1/4". The drum hub reduces the increase to 1/4" or so overall. You'd see 1/2" with the disc hub. Perhaps with the narrow appearing track we see with the "wide track" body combined with the Chevrolet chassis, wider is better. But at the same time, using wider wheels up front with the wider track may cause some rubbing.The fact is the 69' drum hub rotor mounting flange is about 1/8" more inboard. The drum hub is identical, save for the lack of a machined relief in the back where the stock disc would have mounted, uses the same bearings and seal. Yes, the drum or rotor mounting flange has a thinner cross section, but lacking the machined relief in the back actually brings the gusseting further out onto the flange. Shoud be equally as strong. But use your own judgement and choice here.
Disc hub on the left
Some numbers;Disc hub flange =.50" thick x 6.270" dia. Flange face to outboard end = 1 3/4"Drum hub flange=.425" thick x 6.335" dia.Flange face to outboard end = 1 7/8"
Perhaps 1/8 of a inch is no big deal, but at least it brings our wheel in a little closer to stock. It could reduce the chances of rubbing with those running xtra wide front tires. Unless someone can tell me why I should not run the drum hub, this seems pretty logical to me. Keep your drum hubs people!
Note; I have recently learned that using the disc brake hub with the Vette rotor can cause a problem with mounting the caliper. Seems some brake pads are thicker than others, and one will have trouble getting the caliper past and over onto the bracket mounting ears. The disc hub places the rotor 1/8" more outboard, as such an extra thick outboard pad will not allow the caliper to move inboard enough to slip past the bracket. Although conversions have been made successfully with the disc or drum hub, thinner pads or the drum hub will eliminate this potential problem. The Vette rotor centers pretty well in the caliper, pretty close to the stock position really, although the old rotor was pretty worn.
New wheel studs will be required with this conversion. 7/16-20 x 1 1/2. Ask for 69' Vette fronts. 1 3/4 long would be better, but I've not found any listed yet with the same knurl dimentions. If they were 1/2", the options would be greater. Imop, the ultimate would be to drill the rotor and drill and tap the hub for thread in 1/2-20 x 2" studs.
Here's a shot below with the pads in the Calipers. I elected to go with some economy pads for mock up. Raybestos organics. They fit without a problem. One thing to note is that to center the caliper with pads in place, it will move inboard on the sliders about 1/8". This would not be required with the disc hubs. The disc hubs will center the rotor more or less in the caliper a little better. You choose...1/4 less track increase or better centering. I don't think you'd run out of piston before you've worn the pads out regardless.
Here's a summary of the conversion.
Here's some part numbers and the years they cover.
Wheel StudsPapco. 7/16 - 20 x 1 1/2. #560-114. Or Ask for 69' Corvette fronts. They pressed into the Impala hub no trouble.
Corvette Rotors, 65-82 Raybestos, Front Pt # 5500, Rear 5501. Tons more brands and options available here. And far cheaper than the "original" offshore rotor.
Calipers, 69-70 Raybestos, Right Pt# FRC4052, Left FRC4051. More options available, but only 69-70 will fit on our disc spindle setup.
Caliper bolts, 68-76Raybestos, Pt# H5004. Replace if showing any corrosion.
Anti rattle clips, 68-80 Raybestos, Pt# H5408. Fits over the inboard pad and clips into the piston hollow.
Pads, 69-80 Just ask for 69-70 Impala, Many choices. Apparently its virtually the same single piston pad all the way to 80.